1. Field of the Invention
The present invention relates to an image forming apparatus and a method for reading a reference image.
2. Description of Related Art
An electrophotographic image forming apparatus forms a reference image for image quality adjustment on a sheet, changes an image forming condition etc. according to read values obtained by reading the reference image, and adjusts image quality so that it becomes constant.
In the case that curl, waving and/or fluttering of the sheet occur when reading the reference image, a height position of a surface of the sheet sometimes fluctuates. When fluctuation of the height position is large, the read values vary and accuracy of reading is lowered, and accordingly image quality adjustment cannot be performed accurately.
In the case that the reference image is formed in a region where there are few curl and/or waving, accuracy of reading can be prevented from lowering due to fluctuation of the height position of the sheet surface. However, because a usual line sensor used for image reading cannot detect the height position of the sheet surface, the reference image has heretofore been formed also in a region where fluctuation of the height position caused by curl and/or waving of the sheet is large. In order to detect the height position of the sheet surface, a special sensor (for example, see Japanese Patent Application Laid-open Nos. 2007-304492, 2010-266432, and 2012-181555) is necessary.
Meanwhile, in order to suppress fluctuation of the height position of the sheet surface caused by fluttering of the sheet, there have been measures such as providing a guide plate below the sheet so that the guide plate makes a gap through which the sheet can pass narrower at a reading position (for example, see Japanese Patent Application Laid-open No. 2010-145605) and making the sheet contact with a reading surface of the line sensor which reads the reference image.
However, when the gap through which the sheet can pass is made narrower, the resistance of the sheet becomes larger during conveyance. Accordingly, conveyance performance at a constant speed is lowered, and variation of the read values becomes large also in this case.
Moreover, when the sheet contacts with the reading surface of the line sensor, the reading surface is easily soiled. Especially, when an anti-smudge coating layer is provided on the sheet, there is a case that the coating layer comes unstuck due to friction against the reading surface and such condition is read as vertical stripe noises, and accordingly accuracy of reading is significantly lowered.